Conventional disk drives recycle intermittently faulty disk sectors indefinitely. For example, a sector that is tagged as being unreadable but that has not yet been relocated may be reused when the host system overwrites the tagged location and successfully verifies the written data. Indeed, there are disk sectors that are readable most of the time but that may be intermittently unreadable only under certain conditions. Some of these intermittently unreadable sectors may escape the factory defect mapping process. Such disk sectors may be recycled and reused indefinitely, thereby causing the error rate to increase over time. When the error rate reaches a predetermined threshold, the drive's warranty may be triggered.
Conventionally, a disk sector that becomes unreadable after being written may be tagged as a TARE (Transparent Auto Relocation Entry), which establishes a mechanism to enable the drive, under certain circumstances, to re-read and reuse the tagged sector. Once the host system attempts to overwrite the tagged disk sector in an attempt to reuse it, a verification test is performed in which the disk sector is repeatedly written and read to confirm that the tagged sector is, in fact, usable. Unfortunately, some disk sectors are verified to be recycle-worthy when the verification test is carried out, but fail thereafter. Indeed, some sectors may appear to be functional as the signals are refreshed through overwrite during the overwrite test but may lose their integrity after prolonged period of inactivity. These sectors are prone to errors but may, nevertheless be recycled over and over again as long as the verification test is successful.
Data from field returns of disk drives have shown that there are many cases where the same disk sectors are being repeatedly reused and recycled in this fashion, with an incremental increase in the error rate on each read attempt that resulted in an unreadable condition. Conventionally, however, the host system does not appear to take note of the intermittent, albeit sector-consistent, nature of such errors keeps accessing the intermittently faulty disk sector until a check to the SMART Status returns a failure caused by the error rate exceeding the warranty threshold. What are needed, therefore, are methods and data storage devices configured to efficiently handle such intermittent errors.